This class represents a dynamic tag string, whose value is determined by invoking a pre-determined callback, which is described by a Callback object.

You need to make your application depend on the Callback module from CPAN if you make use of this tagging feature, since Log::Agent does not depend on it, on purpose (it does not really use it, it only offers an interface to plug it in). At least version 1.02 must be used.

The callback to invoke to determine the value of the tag. The call is protected via a busy flag, in case there is an unwanted recursion due to a call to one of the logging routines whilst within the callback.